In marine constructions, such as marine vessels and marine structures, a known way to protect an immersed metal part against galvanic corrosion is to provide a sacrificial anode, made of very pure zinc, magnesium, cast iron or an alloy of aluminum, which is directly fastened to, or electrically connected via a cable to the immersed metal part. Thereby, a circuit is provided by an electrical current through the water, serving as an electrolyte, and a surface polarization at the interface between the metal part and the water is created, serving to protect the metal part against corrosion. In such a system with a sacrificial anode, herein referred to as a passive corrosion protection system, or simply a passive system, the sacrificial anode will waste away, preventing damage to the immersed metal part. In some marine corrosion protection systems a passive system is combined with an impressed current cathodic protection (ICCP) system, such as described in U.S. Pat. No. 7,044,075B2.
A problem with passive systems is that the sacrificial anode might be passivated by its surface being covered by a non-conductive oxide layer, (passivation layer), which limits the possible galvanic protection current, so that galvanic corrosion may occur on the metal part which is to be protected. The risk of passivation is specially large if the current drawn from the anode is low for a long period of time, and/or if the salinity of the water is low, such as in the case of marine vessels in brackish or fresh waters.
For marine vessels, a known solution to the stated problem is to lift the boat and remove the passivation layer from the sacrificial anode manually. However, this practice consumes a considerable amount of time and cost. It is also known to use, for sacrificial anodes for boats in fresh water, magnesium which is more reactive than zink or aluminum, and therefore more resistant to passivation. However, magnesium can not be used in salt water, due to over-activity and a very short lifetime.